The present invention relates to the preparation of waste paper stock in general, and more particularly to a method of pulping and grading waste paper so as to produce a waste paper stock therefrom.
The recovery of pulp or stock from waste paper is well known per se. It is known that for this purpose the waste paper must be shredded in a suspension, to form from it a pulp which can be further processed. A difficulty in the preparation of stock from waste paper resides in the fact that quite often substances of low specific gravity, such as synthetic plastic foams, rubber, foils and the like are contained in the waste paper, substances which can be removed from the receptacle wherein the pulping takes place neither through the pulp outlet nor through the outlet provided for removal of heavy substances which are not intended to be used as part of the pulp. The difficulties created by this light-weight matter, hereafter designated for the sake of convenience as "impurities", result from the fact that such matter accumulates very rapidly in the receptacle so that after an operating period of only a few days the apparatus must be shut down to permit a complete emptying of the receptacle. It has been observed that in many instances the operating time of the apparatus, that is the time from one to the next shut-down and complete emptying of the receptacle of the apparatus, is as little as two or three consecutive days.
It goes without saying that a complete shutdown of the apparatus means lost time and increased expenses, and that this is not tolerable. The problem having been realized, the prior art has proposed to withdraw from the receptacle a partial stream on the pulped suspension, to supply it to an additional pulper or disintegrator and to a subsequent coarse grading device, and then to return it into the original receptacle. Such an approach does offer some hope of improvement, but quite evidently it also will increase the expenses involved and decrease the economy of operation of an apparatus of the type here under discussion, due to the investment costs required for the additional equipment and the increased energy requirements for operating such equipment. The problem outlined above thus has not heretofore been satisfactorily solved.